This invention relates to polymeric compositions and specifically to smoke suppressant/flame retardant additives for polymeric compositions comprising halogen-containing polymers. More specifically, this invention relates to a method of improving the flame and smoke suppressing characteristics of halogen-containing polymeric compositions by adding to said compositions effective amounts of at least one inhibitor consisting essentially of an intimate mixture of zinc oxide and zinc salts, i.e. zinc borate, zinc phosphate or zinc sulfide.
Polymeric compositions comprising halogen-containing polymers used for sheeting, synthetic fibers and the like are hazardous due to flammability and it is well known to incorporate within these polymeric compositions certain flame retardants and/or smoke suppressants to reduce both the flammability and smoke. Moreover, these polymeric compositions are known to create large amounts of smoke during burning which is more hazardous then the flame. Thus, there is increased concern for safety due to the fact that smoke evolution during the burning of these plastics is as much a hazard if not more than the flames. Certain additives can be used to avoid the hazards due to smoke and toxic gases. These additives include, for example, antimony oxide either alone or in combination with other compounds known to suppress both flame and smoke in halogen-containing compositions.
Accordingly, it is an object of this invention to provide a smoke suppressant and flame retardant for halogen-containing polymeric compositions comprising an intimate mixture of zinc oxide and certain metal salts of zinc which are more effective than either one of the components alone. Zinc oxide alone has not been found useful as a smoke suppressant or flame retardant particularly in polyvinyl chloride in that it causes rapid degradation during processing. For example, trace amounts of hydrochloric acid in these polymeric resins converts the zinc oxide to the chloride which catalyzes the dehydrochlorination of the resin resulting in a phenomenon known as zinc failure. Zinc failure manifests itself by the appearance of black spots in the plastic film. Continual degradation results in the loss of hydrochloric acid and finally the total destruction of the polymer. To avoid these problems, various stabilizers have been used including, for example, the use of barium or cadium salts such as the stearates to prevent or at least delay zinc failure. However, these stabilizers alone are generally not sufficient to avoid the potentially danger of dehydrochlorination when the polymeric materials are used in high pressure equipment such as extruders, injection molding machines and the like.
Moreover, zinc oxide alone is not recommended as a polymeric additive since in certain flame tests zinc oxide appeared to be too reactive. In some instances, the flames were found to spread in formulations utilizing zinc oxide and in some cases worse then the control which had no fire retardant present in the formula. Moreover, other commercially available additives are more costly and therefore it would be an advantage if zinc oxide could be used to reduce cost at least as a partial if not a total replacement for these flame retardants or smoke suppressants, e.g. molybdum oxides, zinc borates, antimony oxides, etc.